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S Y M P O S I U M : C O M P L I C A T I O N S O F H I P A R T H R O P L A S T Y

Reduced Articular Surface of One-piece Cups

A Cause of Runaway Wear and Early Failure

William L. Griffin MD, Christopher J. Nanson MD,

Bryan D. Springer MD, Matthew A. Davies PhD,

Thomas K. Fehring MD

Published online: 18 May 2010

The Association of Bone and Joint Surgeons1 2010

Abstract

Background Despite the clinical success of modernmetal-on-metal articulations, concerns with wear-related

release of metal ions persist. Evidence suggests metal ion

release is related to the effective coverage of the head in

the metal shell (the cups functional articular arc). A recent

study suggests a reduced functional articular arc is asso-

ciated with increased ion release and the arc is a function of

component design, size, and the abduction angle.

Questions/purposes The purposes of this study were to

(1) measure the functional articular arc in different sizes

of currently available one-piece metal shells from several

different manufacturers; and (2) compare the functional

articular arc of these one-piece metal shells with the

1808 arc of conventional hip arthroplasty acetabular

components.

Methods We calculated the available articular surface arc

for 33 one-piece metal cups using measurements of cupdepth and internal cup radius.

Results The arc of the articular surface varied among

manufacturers and generally decreased with decreasing

shell diameter. The mean functional articular arc was

160.5 3.6 (range, 151.8165.8), which was less than

the 180 arc of a conventional acetabular component.

Conclusions Our data show certain cup designs are at

higher risk for failure as a result of the decreased articular

surface arc. This, along with analysis of abduction angles,

supports the recent findings of bearing failure with verti-

cally placed implants. Care must be taken when implanting

these shells to ensure they are placed in less abduction to

avoid edge loading and the potential for early bearing

failure.

Introduction

Although some of the earliest THA designs incorporated

metal-on-metal bearings, poor understanding of the man-

ufacturing and tribologic needs of these bearings led to

high implant failure rates and, as a result, metal-on-poly-

ethylene became the dominant articular couple for several

decades. Improved metal implant manufacturing tech-

niques with better tolerance, roundness, and metallurgy,

however, have led to recent increases in the use of metal-

on-metal bearings. Metal-on-metal articulations are asso-

ciated with a very low wear rate and as a result of the

tribology of the materials, unlike metal-on-polyethylene

articulations, the wear rate improves as the head size

increases [6, 7, 9, 14]. This combination of low wear and

large head sizes with metal-on-metal bearings has promp-

ted the resurgence of hip resurfacing arthroplasty [3, 18].

One or more of the authors (WG, BS, TF) received funding from

research endowments from The Winkler Fund and The Smith

Arthritis Fund.

This work was performed at The OrthoCarolina Hip and Knee Center

in conjunction with The OrthoCarolina Research Institute and the

Department of Mechanical Engineering, University of North

CarolinaCharlotte, Charlotte, NC, USA.

W. L. Griffin, C. J. Nanson, B. D. Springer, T. K. Fehring

The OrthoCarolina Hip and Knee Center, The OrthoCarolina

Research Institute, Charlotte, NC, USA

M. A. Davies

Department of Mechanical Engineering, University of North

Carolina, Charlotte, Charlotte, NC, USA

W. L. Griffin (&)

1915 Randolph Road, Charlotte, NC 28207, USA

e-mail: william.griffin@orthocarolina.com

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Clin Orthop Relat Res (2010) 468:23282332

DOI 10.1007/s11999-010-1383-8

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Moreover, as a result of the additional stability afforded by

large femoral heads against dislocation, use of one-piece

metal shells with large-diameter heads are now being used

routinely in THA.

Despite the benefits and clinical success of modern

metal-on-metal articulations, concerns with wear-related

release of metal ions persist. Elevated chromium and cobalt

levels have been found in both the serum and remoteorgans of patients with metal-on-metal bearings [1, 2, 10].

Studies have also demonstrated excessive wear with one-

piece metal shells in women with smaller diameter com-

ponents as well as in hips in which the shell is oriented

more vertically in the pelvis (a higher abduction angle)

[5, 11, 13, 16, 19]. Occasionally, more dramatic local

reactions to metal-on-metal articulations occur with a wide

spectrum of presentations; excessive metal wear can lead to

asymptomatic effusions, pseudotumors, and wide zones of

soft tissue necrosis with loss of abductors and bone death

[8, 17].

Evidence has emerged that such elevated metal ionrelease with resurfacing hip arthroplasty is related to the

effective coverage of the head in the metal shell or the

cups functional articular arc. De Haan et al. [5] showed

elevated metal ion release correlates with a reduced func-

tional articular arc, in which the arc is a function of

component design, component size, and the abduction

angle of the cup. They suggested high wear is likely to

occur in metal-on-metal resurfacing arthroplasties if the

acetabular components are implanted steeply, likely as a

result of a greater risk of edge loading, and this is partic-

ularly problematic in patients with a small acetabulum and

in low-profile acetabular components. The authors stressed

surgeons needed to be aware of functional arc of the ace-

tabular component they implant.

Although De Haan et al. [5] have demonstrated the

importance of functional articular arc in the performance of

hip replacement components, it is still not a widely known

concept nor are the data on functional articular arcs readily

available from manufacturers. Moreover, as longer fol-

lowup becomes available for one-piece metal cups, it is

evident some patients develop metal sensitivity reactions

attributable to increased metal wear associated with

variables such as tolerances, material, roundness, as well

as mechanical factors related to cup design. Because of

the correlation with elevated wear and potential higher risk

of failure, knowledge of how these cup characteristics

differ among manufacturers as well as how they differ

from conventional 1808 components is important for hip

replacement surgeons.

The purposes of this study were therefore (1) to measurethe functional articular arc in different sizes of currently

available one-piece metal shells from several different

manufacturers; and (2) to compare the functional articular

arc of these one-piece metal shells with the 1808 arc of

conventional hip arthroplasty acetabular components.

Materials and Methods

New one-piece metal-on-metal acetabular components

were obtained from four different manufacturers in multiple

sizes (Table 1). Eleven cups with sizes ranging from 44 to66 mm were obtained both from Smith and Nephew (Bir-

mingham Hip Resurfacing; Smith and Nephew, Memphis,

TN) and BIOMET (Magnum; BIOMET, Warsaw, IN). Six

cups ranging in size from 42 to 62 mm were obtained from

Wright Medical Technology Inc (Conserve Plus; Wright

Medical Technology Inc, Arlington, TN), and three cups

were obtained from Stryker Orthopaedics (Cormet; Stryker

Orthopaedics, Mahwah, NJ). Two additional cups that are

currently on the market (ASR; DePuy, Warsaw, IN, and

Durom; Zimmer, Warsaw, IN) were retrieved during

revision arthroplasty and included in the analysis. In all,

we studied 31 new and two revision-retrieved one-piece

metal-on-metal acetabular components.

For each component, we calculated the arc of the

available articular surface (Fig. 1) with trigonometry using

measurements of cup depth (measured with a Mitutoyo

Series 129 depth micrometer; Mitutoyo America Corpo-

ration, Aurora, IL) and the manufacturer-supplied internal

radius of the cup. Cup depth was defined as the distance

from the functional rim to the internal dome, taking into

account any cutaways or bevels at the rim. All depth

measurements were repeated 10 times per cup and the

Table 1. Distribution of cups studied

Manufacturer Cup Number of cups Sizes (mm)

Smith and Nephew Birmingham Head Resurfacing (new) 11 44, 46, 48, 50,52, 54, 56, 60, 62, 64, 66

BIOMET Magnum (new) 11 44, 46, 48, 50,52, 54, 56, 60, 62, 64, 66

Wright Medical Technology Inc. Conserve Plus (new) 6 42, 46, 48, 52, 56, 62

Stryker Orthopaedics Cormet (new) 3 52, 56, 62

DePuy ASR (retrieved) 1 48

Zimmer Durom (retrieved) 1 44

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average value recorded and used for articular surface

calculations.

Head size versus measured articular surface arcs was

graphed for each of the manufacturers. For parametric data,

we used a one-sample t-test to compare the mean of a

population to a known number. Because values for articular

surface arc assessed in this study were normally distrib-

uted, we used a one-sample t-test to compare the mean

value for the 33 articular surface arcs in this study to a

fixed value of 1808, the theoretical arc for a conventional

acetabular implant. Owing to the small sample size, we did

not compare differences among manufacturers. Instead, we

assessed whether, as a group, these one piece metal cups

have an articular surface arc that was different from 1808.

We performed the statistical analysis using SPSS software

(SPSS Version 8.0; SPSS Inc, Chicago, IL).

Results

The arc of the articular surface varied among manufac-

turers and generally decreased with decreasing shell

diameter (Fig. 2). The amount of articular surface coverage

ranged from a high of 165 in the largest sizes of two of the

manufacturers designs to 151.8 encountered in the smaller

retrieval specimen. In general, the Conserve cup had the

largest functional arcs across cup sizes, whereas the BHR

and the Cormet had the smallest functional arcs across the

range of cup sizes.

The mean functional articular arc for the 33 cups as-

sessed in this study was 160.5 3.6 (range, 151.8

165.8), which was less than (p\ 0.001) the 180 arc of a

conventional acetabular component.

Discussion

With the increase in the number of resurfacing and THAs

performed with large, nonhemispheric one-piece metal

cups, new complications associated with wear have been

noted. These complications include runaway wear, pseu-

dotumor formation, and acute lymphocytic vascular-

associated lesions [8, 16, 17]. Several reports have recently

been published regarding acetabular component position as

it relates to serum metal ions [5, 11, 13] and cup loosening

[4, 15]. Cups implanted with inclinations ranging from 45

to 60 showed signs of increased edge loading, loosening,

and serum metal ion levels. Additionally, De Haan et al.

have demonstrated the importance of functional articular

arc as it relates to wear complications with large metal-on-

metal articulation [5]. De Haan et al. showed elevated

Fig. 1 The functional articular arc (a) is a function of radius (r) and

depth of the cup (d). The articular arc angle is a measure that is

design-specific. The amount of coverage laterally over the head is a

function of the abduction angle of the cup and the functional articular

surface. Reproduced (in amended form) with permission and copy-

right of the British Editorial Society of Bone and Joint Surgery (De

Haan R, Pattyn C, Gill HS, Murray DW, Campbell PA, De Smet K.

Correlation between inclination of the acetabular component and

metal ion levels in metal-on-metal hip resurfacing replacement. J

Bone Joint Surg Br. 2008;90:12911297, Fig. 2).

Fig. 2 Head size (mm) versus functional articular arc (degrees) is

graphed for different manufacturer shell designs assessed in this

study.

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metal ion release correlates with a reduced functional

articular arc, in which the arc is a function of component

design, component size, and the abduction angle of the cup.

The purpose of the current study, therefore, was to build on

the work of De Haan et al. [5] and (1) to measure the

functional articular arc in different sizes of currently

available one-piece metal shells from several different

manufacturers; and (2) to compare the functional articulararc of these one-piece metal shells with the 1808 arc of

conventional hip arthroplasty acetabular components.

We acknowledge several limitations of the current

study. The primary limitation is that articular surface wear

is a multifactorial process and there are many variables that

affect it which we do not address, including bearing

diameter and cup anteversion. The focus of this study,

however, is functional articular arc. All else being equal,

functional articular arc matters [5] and surgeons should be

more aware of it. Quantification of functional articular arcs

for cups presently on the market may help surgeons

improve their outcomes by enabling them to understandwhich shell designs are at increased risk for edge loading,

specifically if they are positioned suboptimally. Second, we

did not evaluate the full range of implants for each man-

ufacturer. Although we would have preferred to evaluate

many more cups, we were only able to study those cups

donated by the different manufacturers. However, all cups

donated were evaluated and included in the present anal-

ysis. Future analyses should include more cups from

different manufacturers with a complete range of cup sizes.

We observed a wide variation of functional arcs among

currently available manufacturers and cup sizes in which

functional arc generally decreased as shell diameter

decreased. In general, the Conserve cup (Wright Medical)

had the largest functional arcs, whereas the BHR (Smith

and Nephew) and the Cormet (Stryker) had the smallest

functional arcs across the range of cup sizes. Of all the cups

studied, the 44-mm ASR (DePuy) had the smallest func-

tional articular arc. The reason for the variation among

manufacturers is related to design. Design decisions that

compromise hemisphericity are based on many factors,

including retention of bone stock and reducing the risk of

impingement in a resurfacing situation. Some cups are

designed to be less than a hemisphere to prevent

impingement of the native femoral neck against the shell

edge. Some designs reduce hemisphericity by incorporat-

ing a thickened dome to stiffen the shell and lessen the

deformation that can occur during press-fit implantation.

Still other designs incorporate a tapered radius at their rim

to eliminate sharp edges and maintain fluid ingress into the

articulation. Overall, these design differences result in

variability in functional articular arcs. This difference

between designs is clearly demonstrated by a recent clini-

cal report. In a single center study of 660 metal-on-metal

resurfacings, Langton et al. [12] reported 17 failures from

adverse reactions to metal debris in patients implanted with

ASR implants (3.5%) and no failures of this nature in

patients implanted with BHR (Smith and Nephew)

implants. Langton et al. suggest the increased failure of the

ASR cup (DePuy) secondary to the increased generation of

metal debris was the result of its reduced functional artic-

ular arc as compared with that of the BHR (DePuy)component [12].

The articular surface arcs of cups in the current study

averaged 160.6, which was substantially less than the 180

arcs associated with standard THA components. The

smaller bearing surface arc demonstrated with these and

other one-piece metal cups results in a smaller amount of

coverage laterally over the head, which could lead to edge

loading at much lower abduction angles, resulting in

elimination of fluid film lubrication and increased metal

wear. This is consistent with the reports of edge loading

and component loosening as well as increased metal ions

noted in subjects with inclination angles averaging 45 ormore [5, 11, 13, 15]. As previously noted, the amount of

arc available for coverage is a function of the abduction

angle of the cup, the functional arc of the particular cup

design, and the size of the implanted cup. Cups with a

smaller functional articular surface are at higher risk of

edge loading and high wear rates. As an example, if a cup

with a functional articular surface of 151 is implanted at

55 abduction, it will behave like a 180 cup implanted at

69.5 of abduction and be at risk for edge loading. A tra-

ditionally acceptable 45 inclination angle leaves no room

for error in these nonhemispheric cups, particularly in

smaller sizes. This supports the findings of Ollivere et al.

who studied the rate and mode of early failure in 463

Birmingham hip resurfacings [16]. They reported a 3.1%

rate of metallosis-related revision at 5 years with risk

factors for revision including female gender and a high

abduction angle. When combined with excessive antever-

sion, these one-piece cups provide even less coverage of

the weightbearing portion of the femoral head. Given this

possibility, and the increasing number of reports of run-

away wear and loosening as well as pseudotumor formation

and devastating soft tissue necrosis and nerve palsies, it

may be advisable to implant nonhemispheric resurfacing

cups between 408 to 458 of abduction and 158 of antever-

sion and check for impingement intraoperatively. Because

cup position error is not well tolerated with these designs,

an intraoperative radiograph may be advisable to ensure

accurate position.

Our data and analysis of abduction angles are consistent

with recent findings of bearing failure with vertically

placed implants. It is evident certain designs are at higher

risk for failure as a result of the decreased articular surface

arc. Care must be taken when implanting these shells to

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ensure they are placed in less abduction to avoid edge

loading and the potential for early bearing failure.

Acknowledgments We thank Christi Sychterz Terefenko, MS, for

her assistance with the manuscript.

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